1. Field
The present invention relates generally to magnetoresistive devices and, more particularly, to improved methods for fabricating giant magnetoresistive devices, e.g., for magnetic memories.
2. Related Art
The discovery of the giant magnetoresistive (GMR) effect has led to the development of a number of spin-based electronic devices. The GMR effect is observed in certain thin-film devices that are made up of alternating ferromagnetic and nonmagnetic layers. The resistance of a GMR device is lowest when the magnetic moments of the ferromagnetic layers are in a parallel orientation, and the resistance is highest when the magnetic moments are in an antiparallel orientation.
One type of GMR device is commonly referred to as a “spin valve.” A spin valve typically includes two ferromagnetic layers that are separated by a thin layer of a non-magnetic metal, such as copper, and also includes an antiferromagnetic layer that “pins” the magnetization of one of the ferromagnetic layers. Thus, the magnetization of the “pinned” ferromagnetic layer remains generally fixed when moderate magnetic fields are applied. In contrast, the magnetization of the other ferromagnetic layer is relatively “free.” Thus, by applying an appropriate magnetic field, the magnetization of the free ferromagnetic layer can be switched while the magnetization of the pinned ferromagnetic layer is unchanged. In this way, applied magnetic fields can change the relative orientations of the magnetizations in the ferromagnetic layers, which, in turn, can be detected as a change in resistance. In particular, the resistance of spin valve 10 is typically lowest when the magnetizations of the ferromagnetic layers are in a parallel orientation and highest when the magnetizations are in an antiparallel orientation.
Another type of GMR device is commonly referred to as a “pseudo spin valve.” Like a spin valve, a pseudo spin valve typically includes two ferromagnetic layers that are separated by a layer of a nonmagnetic metal, with the magnetization of one of the ferromagnetic layers staying relatively fixed when moderate magnetic fields are applied. However, in a pseudo spin valve, this fixed magnetization is a result of a relatively high switching field rather than a result of being pinned. For example, the fixed ferromagnetic layer may be made substantially thicker than the free ferromagnetic layer.
GMR devices, including spin valves and pseudo spin valves, can be used as data storage elements, i.e., “bits,” in magnetic random access memory (MRAM) devices. In typical MRAM devices, the logical state of a GMR bit is based on its resistance, which, in turn, is based on the relative orientations of the magnetizations of the ferromagnetic layers. Thus, in one logical state, e.g., a “0” state, a GMR bit may have its ferromagnetic layers in a parallel orientation and, thus, may exhibit a low electrical resistance. In the other logical state, e.g., a “1” state, the GMR bit may its ferromagnetic layers in an antiparallel orientation and, thus, may exhibit a higher electrical resistance. Data may be written to a GMR bit by applying a magnetic field sufficient to change the magnetization of the “free” ferromagnetic layer, In this way, the “free” ferromagnetic layer functions as a “switching layer” that stores data in the form of a particular magnetization orientation relative to the other ferromagnetic layer, the “reference layer.”
In general, the process for fabricating GMR bits involves depositing the ferromagnetic layers and other layers that make up the GMR device over one or more underlayers and etching the deposited layers to a desired configuration of GMR bits. Examples of such fabrication processes are described in U.S. Pat. No. 5,496,759 and in U.S. patent application Ser. No. 10/284,922, filed Oct. 31, 2002, which are incorporated herein by reference. However, there continues to be a need for improved methods of fabrication in order to improve yields and/or performance of GMR devices.